Any ophthalmic lens intended to be held in a frame is related to a prescription. The ophthalmic prescription can include a positive or negative power prescription as well as an astigmatism prescription. These prescriptions correspond to corrections to bring to the wearer of the lenses to correct defects of his vision. A lens is fixed in the frame in accordance with the prescription and with the position of the wearer's eyes relative to the frame.
In the simplest cases, the prescription is reduced to a power prescription. The lens is termed unifocal and has a rotational symmetry. It is simply fixed in the frame in such a way that the wearer's main direction of glance coincides with the axis of symmetry of the lens.
For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation for near vision. The prescription thus comprises a far-vision power value and an addition (or power progression) representing the power increment between the far vision and near vision; this amounts to a far-vision power prescription and a near-vision power prescription. Lenses suitable for presbyopic wearers are progressive multifocal lenses, these lenses are described for example in FR-A-2,699,294, U.S. Pat. No. 5,270,745 or U.S. Pat. No. 5,272,495, FR-A-2,683,642, or also FR-A-2,704,327. Progressive multifocal ophthalmic lenses include a far-vision zone, a near-vision zone, an intermediate-vision zone and a substantially umbilical principal progression meridian crossing these three zones. They are generally determined by optimization, based on a certain number of constraints imposed on the different characteristics of the lens. These lenses are all-purpose, in that they are suitable for wearers' differing day-to-day needs.
Families of progressive multifocal lenses are defined, each lens of a family being characterized by an addition which corresponds to the power variation between the far-vision zone and the near-vision zone. More precisely, the addition, labelled A, corresponds to the power variation between a point VL of the far-vision zone and a point VP of the near-vision zone, which are respectively called far-vision control point and near-vision control point, and which represent the points of intersection of the glance and the surface of the lens for far distance vision and for reading vision.
Independently of the power prescription, a wearer may be given an astigmatism prescription. Such a prescription is produced by the ophthalmologist for far vision in the form of a pair formed by an axis value (in degrees) and an amplitude value (in diopters). On a surface, the amplitude value represents the difference 1/R1−1/R2 between the principal curvatures; the axis value represents the orientation, with respect to a reference axis and in a conventional direction of rotation, of the maximum curvature 1/R1. In terms of prescription, the amplitude value represents the difference between the minimum and maximum powers in a given direction and the axis represents the orientation of the maximum power. The term astigmatism is used for the pair (amplitude, angle); this term is also sometimes used, although this is linguistically incorrect, for the amplitude of the astigmatism. The context allows a person skilled in the art to understand which meaning is intended.
Within the same family of lenses, the addition varies from one lens to the other in the family between a minimum addition value and a maximum addition value. Generally, the minimum and maximum addition values are respectively of 0.75 diopter and 3.5 diopters, and the addition varies from steps of 0.25 diopter front one lens to another of the family.
Lenses with the same addition differ by the value of the mean sphere at a reference point, also called a base. It is possible to choose for example to measure the base at the far-vision control point VL. Thus the choice of a pair (addition, base) defines a group or set of aspherical front faces for progressive multifocal lenses. Generally, it is thus possible to define 5 base values and 12 addition values, i.e. sixty front faces. In each of the bases an optimization is carried out for a given power. Starting from semi-finished lenses, of which only the front face is formed, this known method allows to prepare lenses suited to each wearer, by simple machining of a spherical or toric rear face.
Thus, multifocal progressive lenses generally comprise an aspherical front face, which is the face opposite to the wearer of the spectacles, and a spherical or toric rear face, directed towards the wearer of the spectacles. This spherical or tonic face allows the lens to be adapted to the user's ametropia, so that a progressive multifocal lens is generally defined only by the complex surface of its aspherical face. As it is well known, an aspherical surface is generally defined by the altitude of all of its points. A progressive multifocal lens can thus be defined, at any point on its complex surface, by geometric characteristics comprising a mean sphere value and a cylinder value; these surface characteristics of sphere and cylinder are defined in detail hereafter.
In geometric characterization, the points on the complex surface are referenced in relation to an orthonormalized reference frame (X, Y, Z) linked to the front face of the lens and having their origin at the geometric centre (0, 0) of the lens. By convention, the X axis extends horizontally and the Y axis extends vertically when the lens is considered under the conditions when being worn. The Z axis is normal to the front face of the lens and allows the altitude of each point of the complex surface to be plotted. The principal progression meridian generally coincides with the Y axis in the upper part of the lens—in the far vision zone—and can have a nasal convergence in the lower part of the lens—in the near vision zone.
Moreover, a progressive multifocal lens can also be defined by optical characteristics taking into account the situation of the wearer of the lenses. In fact, the optical laws of ray tracings lead to the appearance of optical defects when the rays deviate from the central axis of any lens. Generally, the aberrations known as power defects and astigmatism defects are considered. These aberrations have already been clearly identified in the prior art and improvements have been proposed. For example, document WO-A-98 12590 describes a method for determination of a set of progressive multifocal ophthalmic lenses by optimization. This document proposes to define the set of lenses by considering the optical characteristics of the lenses and in particular the wearer power and oblique astigmatism, under the conditions when being worn. The lens is optimized by ray tracing, based on the conditions when being worn and the chosen object space.
Document FR-A-2,489,971 describes a progressive multifocal lens on which the lateral aberrations in the progression zone are reduced. This document proposes a lens having spherical surfaces for far vision and near vision connected by at least two quasi-isodistant umbilical lines of the principal progression meridian. The central progressive surface is then constructed, limiting the cylinder to 0.25 diopter, this central surface being continuously, connected to the tangent planes of the far-vision and near-vision zones. The lateral surfaces are then constructed, respecting the conditions of continuous connection with the tangent plane of the surface of the channel and the far-vision and near-vision zones.
Document WO 2004/070426 describes a progressive multifocal lens, the horizontal prismatic differentials of which are controlled along the principal progression meridian. In particular, the horizontal prismatic refringence power varies progressively along the meridian independently of the spherical power progression. Such a lens allows improved comfort to be provided to myopic wearers. This document focuses on the control of the lens meridian, the peripheral zones then being optimized according to any known method.